The present invention relates to a process and apparatus for machining by EDM an electrode workpiece by means of an electrode tool, whereby consecutive voltage pulses are applied across the electrodes to cause current pulses of a predetermined intensity to flow through the gap between the electrodes, and wherein the machining condition in the machining zone between the electrodes are monitored and the intensity of the current pulses is increased when short circuits are detected.
It is well known that the short circuits occurring between the electrode workpiece and the electrode tool in the course of EDM machining operation, which are caused by bridging of the electrodes by metallic particles, more particularly during a finishing pass, can be eliminated by burning or melting by means of high current intensity pulses. Such a method is disclosed, for example, in U.S. Pat. Nos. 2,951,969, 3,609,281 and 3,671,705.
However, the methods disclosed in the above-mentioned patents contemplate applying high intensity current pulses as soon as a short circuit is detected in the machining gap, which generally results in applying high energy pulses across the gap. The application of such high energy pulses across the gap in turn causes a rapid deterioration of the machined surface. Consequently, such a method could be used, until now, only during rough machining where the surface finish is not too important and during which short circuits are few. Such a method was consequently practically unusable for finish machining during which low energy pulses are applied across the gap and in the course of which short circuits are many.
The process and apparatus of the present invention have for principal objects to eliminate the inconveniences of the prior art and to make it possible to apply the principle of burning the metallic bridges formed in the machining zone during a finishing pass as well as during a rough cut. It has been observed that the majority of the metallic particle bridges formed in the machining zone are sufficiently weak to be destroyed by burning off simply by the current flow caused by the short circuit itself, during a fraction of the duration of the current pulse, and that the material removing effectiveness of the pulse reappear before the end of the pulse. Under those conditions, which occur most of the time, it is therefore useless to apply a high current pulse as soon as a short circuit is detected. The novel process of the invention permits to considerably reduce the number of short circuit burning auxiliary pulses as a result of causing an increase of the pulse current only after a predetermined time interval, or time delay, from the beginning of the normal current pulse.
The novel process of the invention thus permits to decrease the number of pulses designed to burn off, or melt away, the metallic bridges causing short circuits, and it permits to correspondingly increase the number of effective machining current pulses. A spectacular increase of machining efficiency is thus achieved, as compared to the machining efficiency achieved by the conventional methods which contemplate cutting off the machining pulses and waiting until the metallic bridges are broken by retraction of the electrode tool. A further advantage of the new process of the invention is to obtain an improved stability of machining together with a narrower machining gap, and to substantially reduce the rate of wear of the electrode tool. Finally, the process of the invention permits to maintain a smooth surface finish in the course of finish machining.